1. Technical Field
The present invention relates to breathing gas recirculation.
2. Background Information
One type of recirculation system is a gas reclaim system in which breathing gas is supplied at pressure to the user and the user's exhaust gases are recovered, reprocessed and pumped back to the user. Another type of recirculation system is a closed circuit rebreather system in which as the user breathes each breath is passed through a reprocessing circuit and returned to the user. A further type of recirculation system is a semi-closed circuit system in which part of the exhaust gases are recycled to the user, topped up with fresh breathing gas, and excess gas is expelled from the system.
In the case of a reclaim system the gas is mechanically driven around the breathing circuit; in a rebreather system the breathing action of the user has to drive the gas around the circuit. In all cases it is vital to supply adequate gas to the user without requiring him to exert great effort in breathing. A critical factor in the acceptability of any breathing apparatus to the user is the work of breathing (WOB) required: a high WOB will lead to discomfort and fatigue for the user, and may provide insufficient gas flow during heavy breathing, exertion and the like.
A large part of the WOB of any breathing gas recirculation system is associated with the energy absorbed by changes in gas flow through hoses, valves and other associated parts of the breathing apparatus. Both changes in flow rate and changes in flow direction increase the WOB.
As a user of a breathing gas recirculating apparatus breathes in and out the changes in gas flow cause pressure swings within the system. The present invention aims to minimise the pressure swings in a gas recirculation system and to provide an improved breathing gas flow to and from a user of a recirculation system. The advantages which result from this are a low system pressure drop over the breathing cycle and a low breathing resistance, hence a low WOB. The present invention aims to accommodate changes in gas flow due to the user's breathing action and thus to provide a breathing apparatus which allows a substantially constant gas flow for recirculation.
It is known to include in a breathing gas recirculation system a chamber of variable volume to accommodate breathing gas and which is capable of expanding and contracting in response to displacement of the user's own lungs which causes pressure swings in the breathing circuit. Such chambers are commonly made (at least partly) of a compliant material and are known in the art as counterlungs or breathing bags.